Why are releases compressed into multiple RAR archive parts?

The simple answer, is that this is what the scene rules specify. These rules make it easier to manage files for members of the scene. When transferring data, corruption can occur at any time, so when a file becomes corrupted during transit, for instance, from a topsite to a users computer, it has to be re-downloaded. For very large files, this results in a lot of wasted bandwidth and time. To avoid this, the file is split into several smaller archived parts, which are downloaded individually. If one of these parts is found to be corrupted, all a user must do is re-download that small part.

There is much more debate as to whether these RARs are a good or bad thing when discussed in relation to peer to peer. For this debate, read this article: The RAR Argument.